MARIANA, Brazil—Six days after a dam at their jointly owned subsidiary burst and unleashed a deadly avalanche of mud and water, the heads of two of the world’s biggest mining companies made their first public appearance in Brazil on Wednesday.

But the chief executives of Brazilian mining company Vale SA and Australia’s BHP Billiton Ltd. provided no explanation as to what may have caused the disaster at their joint venture, Samarco Mineração SA.

At a news conference with BHP CEO Andrew Mackenzie held next to Samarco’s sprawling iron-ore mine, Vale CEO Murilo Ferreira said their current priority is to save lives and offer solidarity to victims. After that, he said, the causes of the accident will be fully investigated.

Brazilian prosecutors and local officials have said Vale may have been using Samarco’s reservoir to hold tailings from its nearby iron-ore mines. If that proves to be the case, it could raise questions about overcapacity at the dam system, which Samarco was expanding as it ramped up iron-ore production to offset falling prices.

Vale’s Mr. Ferreira declined to answer questions at Wednesday’s news conference about whether his company had been dumping its own mine waste, known as tailings, into the joint venture’s dam system before it broke.

Minas Gerais state prosecutor Carlos Eduardo Ferreira Pinto said Tuesday that negligence likely played a part in the breach of Samarco’s Fundão and Santarém dams. The accident unleashed more than 60 million cubic meters of water and heavy mud that killed eight people, left another 21 missing, and caused unmeasured environmental destruction to hundreds of kilometers of watersheds downstream.

“No operation of this size just breaks without warning,” Mr. Pinto said Tuesday.

Rescue workers say they have visited all the affected communities and there is little hope of finding victims alive at the scene of the surge. A few people initially reported as missing have turned up elsewhere in the days since the accident.

In addition, Messrs. Mackenzie and Ferreira pledged to support Samarco’s relief efforts by contributing to an emergency fund to help victims and finance rebuilding. Vale and BHP Billiton have both sent health, safety, environment and geotechnical experts to the site.

Mr. Ferreira said it is necessary to unify the companies’ response under Samarco CEO Ricardo Vescovi.

“Samarco has shown itself to be absolutely competent in spite of the devastating scene that occurred,” Mr. Ferreira said.

Corrections & Amplifications: BHP Billiton Chief Executive Andrew Mackenzie held a news conference in Australia last week. An earlier version of this article incorrectly stated that Wednesday’s news conference marked the first public appearances of BHP Billiton’s and Vale’s CEOs since last week’s dam break in Brazil.